1. Field of the Invention
The present invention relates to dispensers for viscous materials and particularly to apparatus for use in the injection of a sealing compound into the taphole of a blast furnace for the purpose of closing the hole. More specifically, this invention is directed to minimizing leakage past the piston of an injector mechanism, for example a clay gun associated with a blast furnace, by insuring a close fit between the injector piston and walls of the cylinder in which it moves under all operating conditions. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in and thus will be discussed in the environment of apparatus employed for sealing the pouring holes of a shaft furnace. Such sealing devices are known in the art as "clay guns". Reference may be had to U.S. Pat. No. 3,765,663, which is assigned to the assignee of the present invention, for a general discussion of the operation of clay guns.
As reported in the referenced patent, the pouring or tapholes of a shaft furnace, for example a blast furnace employed in the production of steel, are plugged by injecting thereinto a sealing compound, for example clay or some anhydrous material, which hardens rapidly. A clay gun is conventionally provided with a hydraulic jack which actuates an ejector piston. The ejector piston of the clay gun slides within a clay chamber and forces material supplied to this chamber through a "nose" which is inserted into the furnace taphole. Modern blast furnaces operate at very high counterpressures and thus the sealing compound; i.e., the clay; must be ejected from the clay gun at a very high pressure. The most practical manner of developing the requisite injection pressure for the sealing compound is to actuate the ejector piston of the clay gun by means of a hydraulic jack.
Continuing with a discussion of the problems associated with clay guns, the harsh operating conditions to which such apparatus is exposed presents operational problems which are exceptionally difficult to overcome. Thus, because of the high temperatures to which the apparatus is exposed and the presence of abrasive particles in the sealing material, clearance must be provided between the ejector piston and the wall of the cylinder in which it moves in order to insure against the binding of the piston as a result of either thermally induced deformations of the cylinder walls or because of the presence of particulate matter between the piston and the cylinder wall. However, the clearance between the piston and cylinder wall must be minimized in the interest of preventing the pressurized sealing material from leaking past the piston. Any sealing material which gets behind the ejector piston may prevent proper operation of the clay gun through accumulation which prevents the free movement thereof and also through scoring the polished surface of the hydraulic jack piston rod thereby resulting in hydraulic fluid leakage.
Prior art clay gun ejector pistons have included one or more piston rings to insure a close fit between the piston and the wall of the clay chamber cylinder in which it moves. In the case of modern furnaces, the clay guns may operate at nominal pressures which exceed 200 kg/cm.sup.2 and at these pressures the sealing action of conventional piston-piston ring arrangements has been found to be inadequate; i.e., the rings do not contact the walls of the cylinder with sufficient force to contain the sealing compound at the high pressures involved.
The use of a single piston ring of comparatively large dimensions and with a strong pre-compression has been unsuccessfully implemented as the solution to the leakage problem. It has also been purposed to employ a single piston ring having a rearwardly facing surface which extends rearwardly from the inner diameter of the ring at an oblique angle relative to the axis of the ejector piston and to provide a complementary surface on the piston. This arrangement results in the ring being forced outwardly during the delivery stroke of the piston as a result of the contact between the angled complementary surfaces on the piston and piston ring. The desired expansion of the piston ring requires that force be exerted on the front face of the ring and, accordingly, the front of the ring must be exposed to the sealing compound. Accordingly, the ring must be received in an annular groove and, in prior art proposals, the ring was held in position in such annular groove by a disc or plate affixed to the front face of the piston; the holding disc or plate thus in part defining the groove in which the piston ring was disposed. In order for the sealing compound to contact the front face of the piston ring, the retaining plate was of smaller dimensions than the cross-sectional area of the piston. Also, the groove which receives the piston ring must necessarily be wider and deeper than the corresponding dimensions of the ring itself. In the prior art, accordingly, sealing compound can pass by the outer edge of the retaining plate and collect in the gap between the front face of the ring and the plate and also between the inner diameter of the ring and the piston. This results in the clay exerting a force on both front and inner faces of the piston ring and these forces tend to induce expansion of the ring on the delivery stroke of the piston.
The above described concept of the use of a piston ring and groove, with complementary surfaces on the rear face of the piston ring and on a forwardly facing surface of the piston, has not solved the problems of achieving a tight substantially leakage free fit between the piston and cylinder of a clay gun which operates at a very high pressure. Thus, the fact that the clay must pass around the retaining plate before acting on the ring reduces the force applied to the front of the ring and thus reduces the expansion resulting from the oppositely directed pressure of the clay and the pressure from the action of the piston at the oblique profile at the rear face of the ring. Also, the clay or other sealing material tends to harden and become encrusted between the retaining disc and the ring and also between the piston ring and the piston thereby progressively reducing the desired expansion effect during the delivery stroke to zero. Further, during the return stroke of the piston the ring is pressed against the retaining disc and, in time, this forwardly directed pressure on the plate results in an unacceptably large clearance between the rear face of the ring and the complementary oblique face of the piston.
As a further disadvantage of the above described technique and apparatus, the clay between the inner diameter of the piston ring and the piston will be forced into the space between the complementary obliquely angled surfaces on the ring and piston and will form a crust in this area. Such a crust, in addition to nullifying the effect of the complementary oblique surfaces on the ring and front face of the piston, may also cause rapid wear of and damage to the ring through the irregular transmission of power from the piston thereto during the delivery stroke.